I tried to cook a frozen pizza and i placed it on the metal tray on a short rack provided in the LG conventional oven. It was burnt despite i reduced the time and temperature on the instruction from the pizza box. I didn't use the rotating ring or the glass plate. Must i use these two even though i am not reheating food or micro-wave the food. PS: don't know how/when to use the trays, glass plate, rotating rings and racks.